The Horizontal Branch Population of NGC 1851 as Revealed by the Ultraviolet Imaging Telescope (UVIT)

Abstract

We present the UV photometry of the globular cluster NGC 1851 using images acquired with the Ultra-violet Imaging Telescope (UVIT) onboard the AS-TROSAT satellite. PSF-fitting photometric data derived from images in two far-UV (FUV) filters and one near-UV (NUV) filter are used to construct colormagnitude diagrams (CMD), in combination with HST and ground-based optical photometry. In the FUV, we detect only the bluest part of the cluster horizontal branch (HB); in the NUV, we detect the full extent of the HB, including the red HB, blue HB and a small number of RR Lyrae stars. UV variability was detected in 18 RR Lyrae stars, and 3 new variables were also detected in the central region. The UV/optical CMDs are then compared with isochrones of different age and metallicity (generated using Padova and BaSTI models) and synthetic HB (using helium enhanced Y 2 models). We are able to identify two populations among the HB stars, which are found to have either an age range of 10-12 Gyr, or a range in Y ini of 0.23 -0.28, for a metallicity of [Fe/H] =−1.2 to −1.3. These estimations from the UV CMDs are consistent with those from optical studies.The almost complete sample of the HB stars tend to show a marginal difference in spatial/azimuthal distribution among the blue and red HB stars. This study thus show cases the capability of UVIT, with its excellent resolution and large field of view, to study the hot stellar population in Galactic globular clusters.

UVIT study of UV bright stars in the globular cluster NGC 4147

“…The UV photometry is a great tool to identify the BS population in a cluster. The data from UV telescopes (HST, GALEX and UVIT) have been excellent tool to explore BS population and their formation scenario (Ferraro et al 1997(Ferraro et al , 2003Dieball et al 2010;Schiavon et al 2012;Gosnell et al 2015;Subramaniam et al 2017;Sahu et al 2019a;Kumar et al 2020Kumar et al , 2021Rani et al 2020). In particular, UVIT observations of star clusters are able to identify the hot binary companions (WD, HB, EHB, etc.)…”

“…The glimpse of ultraviolet (UV) light in the old stellar population of the Galactic globular clusters (GGCs) is dominated by hot luminous UV-bright stars which are mostly the stars of blue horizontal branch (BHB), blue-stragglers (BS), post-asymptotic giant branch (pAGB), and extreme horizontal branch (EHB) phases having temperature more than 7000 K. Various physical properties of these UV-bright populations have been explored using large sample of GGCs observed by Galaxy Evolution Explorer (GALEX, Schiavon et al 2012) and Hubble space telescope (HST, Nardiello et al 2018). Ultraviolet Imaging Telescope (UVIT) on-board the AstroSat satellite (Kumar et al 2012) has also performed the imaging observations of several GGCs in UV with a better spatial resolution than GALEX enabling in resolving the core of the clusters and also in distinguishing UV bright stars of different evolutionary stages to study their physical parameters in a great detail (Subramaniam et al 2017;Sahu et al 2019a;Jain et al 2019;Kumar et al 2020Kumar et al , 2021Rani et al 2020;Singh et al 2020). Using the spectroscopic data of a large number of stars in many GGCs along with the UV photometric observations with HST, the evidence for the presence of multiple stellar populations in GGCs is now well established (see reviews Bastian & Lardo 2018;Gratton et al 2019;Cassisi & Salaris 2020, and references therein).…”

Scientific potential of the Indo-Belgian 3.6-m DOT in the field of Galactic Astronomy

“…In Figure 6, the UVIT FUV image of a massive southern GC NGC 1851, taken by Subramaniam et al (2017), is compared with that of an image taken with the GALEX space mission. The angular resolution of the UVIT image is ∼ 1.…”

UVIT observations of UV-Bright stars in four Galactic Globular Clusters

“…1(b)), for one of the clusters NGC 4590. The stellar populations present in the cluster were separated by identifying their various regions in the UV CMDs previously suggested by Schiavon et al (2012); Subramaniam et al (2017); Sahu et al (2019). We can see that in FUV only BHBs and BSS are Fig.…”